Generally, biomass exists in the form of organics, for instance, grass, wood, crops and residues and wastes thereof. First-generation biofuels were mainly bioalcohol and biodiesel generated from sugar, starch, and vegetable oil. However, the first-generation biofuel faces the problem of competition between raw materials and food. Moreover, the applications of first-generation biofuel are restricted due to a lack of fertile soil capable of supporting the growth of these biomasses.
In order to overcome the first-generation biofuel's shortcomings in terms of raw-material shortage, the second-generation biofuel used lignocelluloses as raw materials. Lignocellulose is capable of growing in a barren land. There is around 22 billion tons of biomass (the energy content thereof is around 45EJ) per year in the world, wherein the lignocellulose accounts for around about 70-95%. Lignocellulose mainly comprises three kinds of ingredients, which are cellulose (40-50%), hemicellulose (25-35%) and lignin (15-20%). Cellulose is the most useful among them when being converted into glucose, and forming bioalcohol through fermentation or bio-platform molecules through dehydration is considered to be the most promising option as a substitute for petroleum-based fuels.
Cellulose is the main ingredient of lignocellulose, which is a polymer generated from combination of glucose monomers through β-1,4 glycosidic bonds. β-1,4 glycosidic bond can be destroyed by acid, causing cellulose hydrolysis and generating compounds such as glucose or oligosaccharides, etc. The first acid used in cellulose hydrolysis to produce sugar was an inorganic acid. However, the existence of these inorganic acids causes problems, including difficulty separating products, reactor corrosion, difficulty reusing the catalyst, and wastewater treatment. These problems urgently need to be overcome. Although a recently developed solid catalyst technology is capable of solving the above-mentioned problems, other problems remain, including a small contact area with reactants, low acidity, and weak mechanical strength. These can result in inefficient cellulose hydrolysis of the solid catalyst, and long-term use may causes a decrease in mechanical strength and lead to collapse.